Ministry of Science & Technology
A new photonic memory developed with multilevel capability for optoelectronic data storage applications
A new photonic, functional memory based on tin oxide slanted nanorod arrays in which both the optical and electrical stimuli can be used to modulate switching characteristics shows potential for developing high-density and high-efficiency computing systems.
Currently, various research groups worldwide are designing and realizing non-volatile, ultrafast, reliable, functional memory systems that outperform traditional silicon-based flash memories. In this big data era, a new class of data storage devices that can overcome the physical limitations of the existing memory technologies is being pursued vigorously. One such class of memories is commonly known as memristor , which can store and process data through electrical signals.
Figure 1.  Cross-sectional FESEM image depicting the SnOx slanted nanorod array.  Schematic of Al/SnOx/FTO device upon light illumination.  Optically controlled resistive switching in Al/SnOx/FTO devices under 365 nm light illumination for several switching cycles demonstrating the electrical SET and optical RESET processes.
Recently, researchers from the Centre for Nano and Soft Matter Sciences , Bangalore, an autonomous institution of the Department of Science and Technology , Govt. of India, have designed such a functional memory based on tin oxide slanted nanorod arrays that shows great potential for the development of high-density and high-efficient computing systems. In this restive memory , both the optical and electrical stimuli can be used to modulate the switching characteristics, including multilevel cell operation.
The CeNS team consisting of Swathi S. P., Athira M., and S. Angappane developed the photonic memory in which the tin oxide slanted nanorod arrays are used as an active layer. The tin oxide nanostructures are prepared by electron-beam evaporation through a technique called the glancing angle deposition  technique.
The electron-beam evaporation is a physical vapor deposition method wherein a focussed electron beam is made to bombard the desired target material, which results in its vaporization, and, eventually, deposition of the target material onto the substrate. GLAD facilitates the preparation of complex nanostructures by manipulating the coordinates  of the substrate.
The researchers observed good switching characteristics of the memory devices, including low operating voltages, moderate ON/OFF ratio  to the OFF state  of the memory device), longer endurance, and better retention with a self-compliance effect in the dark. Interestingly, an unusual negative photo response with an enlarged ON/OFF ratio of greater than 107 and a faster response time is observed under illumination ranging from ultraviolet  to visible light .
The negative photo response is characterized by the decrease of the current in the active layer of the device upon light illumination. They found that these devices can be electrically SET  to LRS and optically RESET  to HRS.
Remarkably, multiple low and high resistance states have been achieved by modulating the programming current and optical stimulus. Moreover, they have presented ample experimental evidence which suggests that the electric field-induced formation and light-induced dissolution of oxygen vacancies are responsible for the optically-stimulated resistance switching. In other words, multiple nanoscale conductive filaments composed of oxygen vacancies  are formed on applying the electrical bias, and the photo-stimulated recombination of the surrounding oxygen ions with the vacancies results in the rupturing of the formed conductive filaments. In this manner, the local conductivity of the tin oxide nanorod array could be modified by the synergistic interplay between the electrical and optical means.
The research recently published in ACS Applied Materials and Interfaces can enable the design and development of photonic memories based on metal oxide nanostructures and help explore their potential applications in artificial visual memory and optoelectronics.
Publication link: https://doi.org/10.1021/acsami.2c22362
For more details, Dr. S. Angappane  can be contacted.
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